Uruguay I would say 3 is enough, the country just doesn't have that many big cities with people to fill those stadiums with. Two in Montevideo and sorroundings (New Centenario should be one, the other one maybe could be an enlargment of Nacional's stadium), another one in... hmm... Punta del Este maybe? it's the biggest vacational city of the country.
In Argentina I would imagine renovations in: Buenos Aires, Cordoba, Rosario, La Plata, Mendoza, Santa Fe (This 6 cities host for sure)
Then new-builts in: maybe Mar del Plata, one in the northwest (San Juan, Salta or Jujuy), one in northeast (Corrientes or Entre Rios), and maaaaaybe one in the south, though I doubt because it's cold as f*ck in June/July.